1. Field of the Invention
The present invention relates to a data communication system for executing transmission and reception of digital data by modulating digital data for each bit thereof, by using a code modulation according to a CDMA (abbreviation for code division multiple access) technique in the field of optical communication utilizing light or the like.
A communication system in which a signal produced by multiplying a data stream of a digital form, which is an object of transmission, by a spreading code having a sufficiently long cycle, such as a pseudo-random signal for every bit, is used to modulate an intermediate-frequency signal (IF signal) or a carrier signal, has been widely adopted as a spread spectrum direct sequence based on the CDMA in the field of radio communication or the like. According to the spread spectrum direct sequence, a transmitting side uses the spreading code to modulate original transmission data according to PSK (abbreviation for phase shift keying) or the like, transmits the transmission data in the form of a radio wave. A receiving side demodulates data according to the PSK so as to synchronize and correlate the data with the spreading code used to transmit each signal (each bit) representing the transmission data, and thus reproduces the original transmission data.
The present invention utilizes the fact that optical communication using light enables much freer setting of the bandwidth occupied by data than radio communication does. Further, the present invention refers to measures which are to be taken to cope with various problems underlying multimedia or on-demand communication and multi-protocol communication by diversely applying the same code spread technique as the technique based on CDMA.
2. Description of the Related Art
For better understanding of the problems underlying a known data communication system utilizing light, an example of the configuration of the known data communication system will be described with reference to FIG. 1.
FIG. 1 is a block diagram schematically showing the configuration of a known data communication system utilizing light. As an example of the known data communication system for transmitting data by performing optical communication over optical fibers, a telephone exchange system, configured in such a manner that links of various hierarchical levels realized with a plurality of optical fibers (for example, low-speed links, middle-speed links, and high-speed links) branch out from a digital switching center, is shown.
In the telephone exchange system shown in FIG. 1, one digital switching center 90 for controlling all communication apparatuses connected on the links of a plurality of hierarchical levels on a centralized basis, is installed in the center of the telephone exchange system. A plurality of middle-speed links ML and high-speed links HL over which serial data is transmitted at a relatively high speed are linked to the digital switching center 90 in such a manner that they branch out from the digital switching center 90. Furthermore, a plurality of links of another hierarchical level, for example, low-speed links LL branch out from each of the middle-speed links ML and high-speed links HL. A plurality of communication apparatuses for subscribers (for example, first and second communication apparatuses for subscribers 92-1 and 92-2) each including a communication equipment such as an individual subscriber's telephone set is connected to each low-speed link LL.
In the telephone exchange system having the foregoing components, generally, the length of a bit stream of serial data to be transmitted over a link of a certain hierarchical level is different from the length of a bit stream of serial data to be transmitted over a link of another hierarchical level. For transferring data from the link of a certain hierarchical level (for example, a middle-speed link ML) to the link of another hierarchical level (for example, a low-speed link LL), it is necessary to exchange data in units of a bit stream, that is, a packet, at each branch point located between the link of a certain hierarchical level and the link of another hierarchical level. For example, in a telephone exchange system shown in FIG. 17, an ATM (abbreviation for asynchronous transfer mode) exchange composed of a plurality of ATM switches including first and second ATM switches 91-1 and 91-2 for adjusting the length of a packet containing data so that the packet can be transferred between links of different hierarchical levels, is installed at each branch point.
As mentioned above, in a known data communication system utilizing light, for example, the telephone exchange system configured in such a manner that links of various hierarchical levels ranging from high-speed links to low-speed links branch out, it is necessary to install an ATM exchange, which is composed of a plurality of switches for adjusting the length of a packet containing data so that the packet can be transferred between links of difference hierarchical levels, at each branch point between links of different hierarchical levels.
In the ATM exchange, since the length of a packet containing data is different depending on communication types, i.e., depending on whether a network (communication network) is composed of high-speed links for high-speed communication or composed of low-speed links for low-speed communication, it is necessary to carry out a complex management concerning data communication. More specifically, priorities concerning data transfer must be determined for each communication type, or steady-state real-time transmission must be guaranteed for data. This causes the problem that the circuitry including a plurality of switches used to achieve the above management or control procedures for controlling the switches becomes complex. Furthermore, after the length of a packet containing data is adjusted in the ATM exchange, even if a data communication speed changes, it is difficult to change the length of the packet containing data. There is therefore a difficulty in flexibly coping with communication requests issued from various kinds of communication equipment installed within the telephone exchange system.
In particular, when a place in which data should be received is a remote place, it is necessary to use many ATM exchanges each having complex circuitry for transmitting data to the remote place. The known telephone exchange system has therefore the problem that an area occupied by the ATM exchanges is relatively large.
On the other hand, from the viewpoint of guaranteeing real-time transmission, a storage device that operates at high speed must be installed for temporarily holding transmission data in an ATM exchange so as to prevent collisions of packets. This makes the circuitry of the ATM exchange more complex. Furthermore, some packet is awaited many times by numerous ATM exchanges until data contained in the packet reaches the remote place. From this viewpoint, there is a difficulty in avoiding a data delay having an uncertain period of time.